If lawyers believe they have a strong legal case, they typically want to get up and argue it without bothering with witnesses in these types of hearings, so the request perhaps indicates the FBI isn't as comfortable as it was in relying solely on legal arguments, an Apple lawyer said.

Apple will present two witnesses: Erik Neuenschwander, its leading cryptography expert, and Lisa Olle, the Apple attorney who dealt with the FBI as it requested details from the phone.

The company plans to argue what it's been saying all along: that the All Writs Act, the late 1700s law the government has based its arguments on, doesn't allow investigators to push Apple to develop custom software that would give the FBI unlimited chances to break the passcode on the iPhone.

Some of the details surrounding Apple's dealings with the Justice Department in the case have been deemed confidential, but Apple lawyers said they hoped those restrictions would now be lifted because of the request for testimony.

The hearing is scheduled to begin at 1 p.m. PDT on Tuesday in District Court for the Central District of California, and it is expected to run until around 5 p.m.